1. Field of the Invention
The present invention relates to a dynamic pressure type bearing and a spindle unit employing the dynamic pressure type bearing, both used in a precision mechanical equipment, e.g., office equipment, acoustic apparatus or measuring apparatus.
2. Description of the Related Art
Hitherto, the present applicant proposed prior-art dynamic pressure type bearings disclosed in Examined Japanese Application Nos. SHO 63-203916 (U.S. Pat. No. 4875,263), SHO 63-243521 and SHO 63-251626.
Examined Japanese Patent Application No. SHO 63-203916 discloses a prior-art dynamic pressure type bearing having a hollow outer cylinder made of metal and a hollow inner cylinder fastened to the inner cylindrical surface of the outer cylinder and made of a thermosetting resin. The inner cylindrical surface of the inner cylinder defines a dynamic pressure producing groove therein. The inner cylinder is fastened to the outer cylinder during molding of the inner cylinder. When the volume of the inner cylinder shrinks in the molding thereof, the bore diameter of the inner cylinder becomes larger since the outer diameter of the inner cylinder remains constant. Thus, a mold defining the inner cylindrical surface of the inner cylinder is readily released therefrom, so that the dynamic pressure producing groove is protected from damage.
However, since a thermosetting resin has a poor wear resistance and frictional characteristic, a dynamic pressure type bearing directly employing the thermosetting resin as a material for the inner cylinder thereof cannot be expected to have a high durability. In addition, the high molding shrinkage percentage of the thermosetting resin will not provide a high molding accuracy.
In accordance with the prior art, a solid lubricant, e.g., a fluorocarbon polymer (e.g., PTFE), has been mixed with the thermosetting resin used for the inner cylinder in order to increase the wear resistance of the inner cylinder. However, the mixture of the solid lubricant causes the molding shrinkage percentage of a molded product made of the thermosetting resin to increase so as to further decrease the molding accuracy of the molded product. Thus, the mixture of the solid lubricant cannot provide an effective solution to increase the durability and the molding accuracy of the molded product.
In particular, since the bore diameter of an air dynamic pressure type bearing is critical, a high molding shrinkage percentage is fatal. In addition, a dynamic pressure type bearing of a poor wear resistance and frictional characteristic cannot be employed for an air dynamic pressure type bearing which employs no lubricant.
In addition, since the poor frictional characteristic of the inner cylindrical surface of the inner cylinder of the bearing produces a large friction on a mating rotatable shaft, this bearing cannot be used with a rotatable shaft made of a light metal, e.g., an aluminium alloy, which is light and inexpensive; but soft to damage.